The development of GMail Counter has been suspended due to a lack of time to dedicate to the project.
I may reconsider this decision sometime in the future, but now I simply don't have enough time to spend on GMail Counter.
If you're interested in the development of this extension, please contact me on Twitter or by email.
I'll consider every proposal, but in the meantime GMail Counter's source code is available on Github for you to fork it.